Reliability of Urinary Albumin, Total Protein, and Creatinine Assays after Prolonged Storage

Abstract

This study investigated the effect of long-term storage at -70 degrees C on urinary albumin, protein, and creatinine measurements in the Family Investigation of Nephropathy and Diabetes, a multicenter study designed to identify genes for diabetic nephropathy. Spot urine samples were collected at eight centers and shipped overnight on ice packs to a central laboratory. Samples were aliquotted and frozen at -70 degrees C for a median of 8 d before initial assay. As part of quality control procedures to determine interassay variability, 351 replicate samples were retrieved from storage at -70 degrees C after a median storage time between original and quality control analyses of 126 d (range 28 to 869 d). Freezer time was characterized as the difference in days between the initial assay and quality control assay. Percentage difference [(quality control - original/original) x 100%] between samples was regressed on storage time and adjusted for original value, age, race, gender, hypertension, and diabetes. After adjustment, freezer time per 30 d was associated with small decreases in percentage difference of urinary albumin (0.25%, P = 0.02), total protein (0.23%, P = 0.02), and albumin-to-creatinine ratio (0.34%, P = 0.001). Urinary creatinine levels were not affected by freezer time (P = 0.25). Measurements of urinary albumin, total protein, and albumin-to-creatinine ratio are minimally affected by storage at -70 degrees C for approximately 2.5 yr. Prolonged storage results in small decreases of urinary albumin and protein that do not substantially affect phenotype classification of overt renal disease.